1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet, for use in, for example, motors and actuators of various types. More particularly, the present invention relates to a method for producing an iron-based rare earth alloy nanocomposite magnet including multiple ferromagnetic phases.
2. Description of the Related Art
Recently, it has become more and more necessary to further improve the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electronic equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence Br of about 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot achieve the high remanence Br of about 0.5 T or more.
An Sm—Co based magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet that achieves the high remanence Br of about 0.5 T or more. However, the Sm—Co based magnet is expensive, because Sm and Co are both expensive materials.
Examples of other high-remanence magnets include an Nd—Fe—B based sintered magnet produced by a powder metallurgical process and an Nd—Fe—B based rapidly solidified magnet produced by a melt quenching process. An Nd—Fe—B based sintered magnet is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Nd—Fe—B based rapidly solidified magnet is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance.
The Nd—Fe—B based sintered magnet is mainly composed of relatively inexpensive Fe (typically at about 60 wt % to about 70 wt % of the total weight), and is much less expensive than the Sm—Co based magnet. Nevertheless, it is still expensive to produce the Nd—Fe—B based sintered magnet. This is partly because huge equipment and a great number of manufacturing and processing steps are required to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for about 10 at % to about 15 at % of the magnet. Also, a powder metallurgical process normally requires a relatively large number of manufacturing and processing steps by its nature.
Compared to an Nd—Fe—B based sintered magnet formed by a powder metallurgical process, an Nd—Fe—B based rapidly solidified magnet can be produced at a lower cost by a melt quenching process. This is because an Nd—Fe—B based rapidly solidified magnet can be produced through relatively simple process steps of melting, melt quenching and heat treating. However, to obtain a permanent magnet of melt-quenched materials in bulk, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
For these reasons, an Nd—Fe—B based rapidly solidified magnet produced by a melt quenching process has a remanence Br that is lower than that of a magnetically anisotropic Nd—Fe—B based sintered magnet produced by a powder metallurgical process.
As disclosed in Japanese Laid-Open Publication No. 1-7502, a technique of adding, in combination, at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr to the material alloy effectively improves the magnetic properties of an Nd—Fe—B based rapidly solidified magnet. When these elements are added to the material alloy, the magnet has increased coercivity HcJ and anticorrosiveness. However, the only known effective method of improving the remanence Br is increasing the density of the bonded magnet. Also, where an Nd—Fe—B based rapidly solidified magnet includes a rare earth element at about 6 at % or more, a melt spinning process, in which a melt of its material alloy is ejected against a chill roller, has often been used in the prior art to rapidly cool and solidify the material alloy at an increased rate.
As for an Nd—Fe—B based rapidly solidified magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1998, pp. 669–670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd3.8Fe77.2B19, where the subscripts are indicated in atomic percentages) and an Fe3B type compound as its main phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which a soft magnetic Fe3B phase and a hard magnetic Nd2Fe14B phase coexist and in which crystal grains of very small sizes (typically on the order of several nanometers) are distributed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a “nanocomposite magnet”. It was reported that such a nanocomposite magnet has a remanence Br as high as about 1 T or more. But the coercivity HcJ thereof is relatively low, i.e., from about 160 kA/m to about 240 kA/m. Accordingly, this permanent magnet material is applicable only when the operating point of the magnet is about 1 or more.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, Japanese Laid-Open Publication No. 3-261104, U.S. Pat. No. 4,836,868, Japanese Laid-Open Publication No. 7-122412, PCT International Publication No. WO 003/03403 and W. C. Chan et. al., “The Effects of Refractory Metals on the Magnetic Properties of α-Fe/R2Fe14B-type Nanocomposites”, IEEE Trans. Magn. No. 5, INTERMAG. 99, Kyongiu, Korea, pp. 3265–3267, 1999. However, none of these proposed techniques are reliable enough to always obtain a sufficient “characteristic value per cost”. More specifically, none of the nanocomposite magnets produced by these techniques achieve a coercivity that is high enough to actually use it in various applications. Thus, none of these magnets can exhibit commercially viable magnetic properties.